Kimmitt was born in Fort Sill, Oklahoma while his father was stationed there. His older brother Robert also served in the U.S. Army and was the former United States Ambassador to Germany. Their father Joseph was the Secretary of the United States Senate and Secretary for the Majority from 1977 to 1981.

Kimmitt retired from the US Army a Brigadier General in December 2006.[7] While in the Army, Kimmitt had command and staff assignments throughout the United States, Europe, Asia and the Middle East, including planning positions within both Allied and Joint service commands. Additionally, he spent three years as an assistant professor and instructor with the Military Academy’s Department of Social Sciences.[5]

Subsequent to retiring from the Army in 2006, Kimmitt served as the Deputy Assistant Secretary of Defense for Middle East Policy, responsible for military policy development, planning, guidance and oversight for the region.[8] Kimmitt served in this position until July 31, 2008, and was sworn as Assistant Secretary of State on August 8, 2008.[8] He left office in January 2009.

Kimmitt's nomination for Assistant Secretary of State was delayed because of two investigations into anonymous letters sent to the Senate Foreign Relations Committee.[7][9][10] Upon receipt of each letter, Sen. Joseph Biden requested that the Inspector General of the Department of Defense (IG) investigate the allegations.[7][9][10] After a four-month investigation, the IG concluded after the first investigation that "BG Kimmitt's leadership style was occasionally inconsistent with the standards expected for senior Government leaders" and that "cognizant management officials should continue to monitor his leadership style."[7] "[T]estimony indicated that morale In BG Kimmitt's organization was negatively affected by BG Kimmitt's leadership style, combined with the heavy workload and long hours. Finally, we found that BG Kimmitt's leadership style discouraged subordinates from free and open communication with him."[7][10] The report also stated that the IG "obtained testimonial evidence that tended to mitigate the adverse impact of BG Kimmitt's leadership lapses. In that regard several witnesses, primarily BG Kimmitt's superiors, emphasized that BG Kimmitt brings superb qualifications and intellect to his position; that he has strengthened the overall performance of his office; and that he operates in a stressful, demanding environment, which could trigger confrontation."[7] The Department of Defense Inspector General's office, in a separate letter to the committee, also disclosed "a substantiated allegation that Mr. Kimmitt . . . failed to properly safeguard information, in violation of Army regulations," but it did not elaborate.[7][10] The exact nature of this violation has not been disclosed.[7]

In the aftermath of the Mukaradeeb wedding party massacre, Kimmitt claimed, "There was no evidence of a wedding: no decorations, no musical instruments found, no large quantities of food or leftover servings one would expect from a wedding celebration. There may have been some kind of celebration. Bad people have celebrations, too." However video footage obtained by the Associated Press seems to contradict this view. The video shows a series of scenes of a wedding celebration, and footage from the following day showing fragments of musical instruments, pots and pans and brightly colored beddings used for celebrations, scattered around a destroyed tent.[11][12]